ENCYCLOPEDIA

HORCUS (Horkos), the personification of an oath, is described by Hesiod as the son of Eris, and the avenger of perjury. (Theog. 231, Op. 209 ; Herod. vi. 86. § 3.)

Source: Dictionary of Greek and Roman Biography and Mythology.

Hesiod, Theogony 226 ff (trans. Evelyn-White) (Greek epic C8th or C7th B.C.) :
"But abhorred Eris (Strife) bare painful Ponos (Toil), and Lethe (Forgetfulness), and Limos (Starvation), and the Algea (Pains), full of weeping, the Hysminai (Fightings) and the Makhai (Battles), the Phonoi (Murders) and the Androktasiai (Man-slaughters), the Neikea (Quarrels), the Pseudo-Logoi (Lies), the Amphilogiai (Disputes), and Dysnomia (Lawlessness) and Ate (Ruin), who share one another's natures, and Horkos (Horcus, Oath) who does more damage than any other to earthly men, when anyone, of his knowledge, swears to a false oath."

Hesiod, Works & Days 218 ff :
"The better path is to go by on the other side towards justice; for Dike (Justice) beats Hybris when she comes at length to the end of the race. But only when he has suffered does the fool learn this. For Horkos (Horcus) keeps pace with wrong judgements."

Hesiod, Works and Days 804 ff :
"Beware of all the fifth days [of the month]; for they are harsh and angry; it was on the fifth, they say, that the Erinyes (Furies) assisted at the bearing of Horkos (Horcus, Oath), whom Eris (Strife) bore, to be a plague on those who take false oath."

Aesop, Fables 170 (from Chambry 298) (trans. Gibbs) (Greek fable C6th B.C.) :
"A certain man took a deposit from a friend but intended to keep it for himself. When the depositor then summoned him to swear an oath regarding the deposit, he realized the danger he was in and prepared to leave the city and go to his farm. When he reached the city gates, he saw a lame man who was also on his way out of town. He asked the man who he was and where he was going. The man said that he was the god named Horkos (Horcus, Oath) and that he was on his way to track down wicked people. The man then asked Horkos (Oath) how often he revisited each city. Horkos replied, ‘I come back after forty years, or sometimes thirty.’
Accordingly, on the very next day the man did not hesitate to swear an oath that he had never received the deposit. But then the man ran into Horkos, who dragged him off to the edge of a cliff. The man asked Horkos how he could have said that he wasn't coming back for another thirty years when in fact he didn't even grant him a single day's reprieve. Horkos explained, `You also need to know that if somebody intends to provoke me, I am accustomed to come back again the very same day.'"

Herodotus, Histories 6. 86c (trans. Godley) (Greek historian C5th B.C.) :
"Glaukos [historical figure] journeyed to Delphoi to question the oracle. When he asked the oracle whether he should seize the money under oath [that is, money entrusted to him which he had sworn to return], the Pythian priestess threatened him in these verses: ‘Glaukos son of Epikydes, it is more profitable now to prevail by your oath and seize the money. Swear, for death awaits even the man who swears true. But Horkos (Horcus, Oath) has a son, nameless; he is without hands or feet, but he pursues swiftly, until he catches and destroys all the family and the entire house. The line of a man who swears true is better later on.’
When Glaukos heard this, he entreated the god to pardon him for what he had said. The priestess answered that to tempt the god and to do the deed had the same effect."

Virgil, Georgics 1. 276 ff (trans. Fairclough) (Roman bucolic C1st B.C.) :
"Luna (the Moon) herself has ordained various days in various grades as lucky for work. Shun the fifth; then pale Orcus [Horkos] and the Eumenides [Erinyes, Furies] were born."
[N.B. Virgil uses the Latin name Orcus for Horkos. Cf. Hesiod's Works and Days 804ff above.]